(1) Field of the Invention
The present invention relates generally to power amplifier control and, more particularly, to a high isolation radio frequency (RF) power amplifier having a PIN diode attenuator control circuit for substantially eliminating self-biasing of bipolar devices associated with the RF power amplifier under large signal drive conditions.
(2) Description of the Prior Art
Consumer demand for wireless communication is ever increasing worldwide, placing a burden on the existing cellular infrastructure. In Europe, the increased demand led regulators to create the DCS cellular telephone band. DCS utilizes GSM baseband technology at canier frequencies near 1.8 GHZ.
Also in response to consumer demand, cellular telephone manufacturers strive to reduce the size and weight of their phones. With the introduction of DCS, and the creation of dual band GSM/DCS phones, the quest for ever smaller components has intensified. Manufacturers frequently specify the use of a 3-volt battery, either a single lithium ion cell, or a plurality of nickel/metal hydride (NiMH) cells to help reduce phone size and weight. With such a power source, manufacturers also specify various specifications for certain performance characteristics. Some of the most important characteristics are associated with low voltage operation, power, high efficiency, small size and power control. For example, many manufacturers presently specify that the DCS power amplifier must be capable of operating with the battery voltage as low as 2.7 volts, with a nominal voltage near 3.2 volts. This specification allows greater utilization of the battery charge to maximize talk time while complying with DCS standards. In view of the foregoing manufacturing specifications, it is desirable in the industry to provide a workable DCS power amplifier utilizing bipolar device technology.
Isolation of bipolar device power amplifiers under large signal drive conditions however, has been particularly problematic. For example, when the control voltage to a power amplifier is at 0 volts, the amplifier should be turned off. During this off condition, the power amplifier generally is required to have a specified minimum isolation between its input and its output. Self-biasing of specific bipolar devices can occur under certain large signal drive conditions. This self-biasing can reduce the aforesaid isolation by turning on one or more stages of a bipolar device implemented power amplifier. Techniques have been developed that attempt to reduce or eliminate the degree of self-biasing described above. Using power amplifiers constructed with MESFET technology help to minimize this problem since they are less susceptible to self-biasing as compared with power amplifiers constructed with bipolar devices. Another technique used to reduce or eliminate the aforesaid self-biasing includes the use of discrete devices to enhance the degree of isolation per stage. Other techniques have also been utilized, including use of a RF switch at the input to the power amplifier.
Although the foregoing techniques may have proven successful in certain limited applications, these techniques are not as desirable in applications specific to integrated circuit design where size and cost constraints are of paramount importance. For example, discrete designs simply are not feasible when a system application necessitates sole use of integrated circuit structures. Additionally, certain technologies, e.g., GaAs HBT technology requires fewer external components than a MESFET design and generally has higher efficiency as well. Further, MESFET's tend to require external switches to be turned off completely under certain operating conditions. Use of such external switches is undesirable when associated with integrated circuit structures since they can add to component count, size and cost.
Thus, there remains a need for a new and improved integrated circuit power amplifier control circuit suitable for use with bipolar device power amplifiers and that effectively minimizes power amplifier self-biasing under large signal drive conditions thereby ensuring optimum isolation between the power amplifier input point and the power amplifier output point, regardless of the amplifier large signal drive level.